1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting/receiving a Random Access CHannel (RACH) in a communication system, and in particular, to a method and apparatus for transmitting/receiving a Contention Resolution (CR) channel in RACH transmission.
2. Description of the Related Art
RACH is a channel that terminals, or User Equipments (UEs), having no dedicated channel, use to transmit uplink data. The UEs that do not have a dedicated channel are represented by UEs in Cell_FACH, Cell_PCH, URA_PCH or Idle mode in Universal Mobile Telecommunications System (UMTS). Physical RACH (PRACH) can be defined as a set of wireless resources used for RACH transmission. The wireless resources of a PRACH are composed of the following:
1. Preamble Scrambling Code: a scrambling code that is mapped to a particular PRACH on a one-to-one basis. The preamble and RACH data transmitted in a reverse direction for use of the PRACH are transmitted after being scrambled by the preamble scrambling code.
2. Signature Set: a set of Orthogonal Variable Spreading Factor (OVSF) codes with Spreading Factor (SF)=16 at which a maximum of 16 OVSF codes can be allocated per PRACH. The signature set is used for coding preamble and RACH data.
3. Access Slot Set: composed of 2 time slots, and the preamble transmission starts at a start point of each access slot.
FIG. 1 illustrates a RACH transmission method for initial system access in a 3rd Generation Partnership Project (3GPP) system.
In FIG. 1, blocks 101, 102 and 103 are a UE, a Node B, and a Radio Network Controller (RNC). The Node B (NB) manages cells, or a Base Station Transceiver (BTS) for directly participating in communication with the UE. The RNC controls multiple NBs and radio resources, respectively.
Referring to FIG. 1, in steps 111 and 112, the UE 101 and the NB 102 perform a PRACH procedure for RACH message transmission.
The UE 101 conducts a persistence value test using a persistence value mapped to a corresponding Access Service Class (ASC). The persistence value, a real value between 0 and 1, essentially represents a probability that the persistence value test will pass. For example, if the persistence value is 0.5, a success probability of the persistence value test is 50%. The UE 101, when it passes the persistence value test, transmits a preamble to the NB 102 in step 111. The UE 101 randomly selects one of the available signatures mapped to the ASC, encodes the preamble using the selected signature, sets initial power, and transmits the coded preamble at the set initial power. The setting of the initial power is disclosed in detail in 3GPP Technical Specification (TS) 25.331, so a detailed description thereof will be omitted herein.
In step 112, the NB 102 transmits an Acquisition Indication Channel (AICH) signal to the UE 101 in response to the preamble that the UE 101 transmitted in step 111. The AICH signal is used for reporting successful receipt of the preamble signal to the UE 101 that transmitted a particular signature, and also approving message transmission over a RACH.
FIG. 1 represents the case where the NB 102 has successfully received the preamble transmitted by the UE 101. Another PRACH procedure is described in detail in 3GPP TS 25.214, so a detailed description thereof will be omitted.
Upon receiving from the NB 102 an ACK for the preamble transmitted by the UE 101 in the PRACH procedure of steps 111 and 112, the UE 101 transmits an Radio Resource Control (RRC) CONNECTION REQUEST message using an RACH message in step 121. The RRC CONNECTION REQUEST message is a message over which the UE 101 sends a request for an RRC connection to the RNC 103.
After the RRC connection setup is completed, a Signaling Radio Bearer (SRB) can be set up between the UE 101 and the RNC 103, and the UE 101 reports its capability information to the network. The UE 101 can receive from the RNC 103 the basic information including temporary IDentity (ID) information to be used in the cell by the UE 101, like Cell-Radio Network Temporary Identity (C-RNTI). That is, the RRC CONNECTION REQUEST message of step 121 is a message over which the UE 101 sends a request for the RRC connection setup to the RNC 103. Here, unique ID information of the UE 101 can be included in the RRC CONNECTION REQUEST message. An RRC CONNECTION SETUP message of step 122 is a response message to the RRC CONNECTION REQUEST message, and over this message, the RNC 103 can assign, to the UE 101, an intra-cell temporary ID to be used in the cell by the UE 101, a scrambling code to be used by the UE 101, SRB configuration information, and the like. The temporary ID of the UE 101 refers to an ID of the UE 101, to be temporarily used in the cell, instead of a unique ID, or International Mobile Subscriber Id (IMSI), of the UE 101. The unique ID of the UE 101 is not used in a radio interface because of security issues or large size. Therefore, the temporary ID of the UE 101 assigned in the cell is used instead of the unique ID.
There are several possible temporary IDs of the UE 101, used in the cell. For example, there are HSDPA Radio Network Temporary Id (H-RNTI) used for High Speed Data Packet Access (HSDPA), E-DCH Radio Network Temporary Id (E-RNTI) used for Enhanced Uplink Dedicated CHannel (EUDCH), Cell-Radio Network Temporary Id (C-RNTI) used for signaling/data transmission of the UE 101 in the cell, and the like. The scrambling code of the UE 101, a unique code assigned to the UE 101 in a Code Division Multiplexing (CDM) system, is used as a code with which a network node can distinguish the UE 101 in uplink transmission.
An RRC CONNECTION SETUP COMPLETE message of step 123 is a confirmation message of the UE 101 for the RRC CONNECTION SETUP message, and through this message the UE 101 reports to the RNC 103 its available capability information that the RNC 103 can request.
The message transmission from steps 121 to 123 is a message flow for setting up the RRC connection, and shown in FIG. 1 is for an example where RRC connection is successfully set up.
FIG. 2 illustrates a collision occurring as more than two UEs select and transmit the same preamble in the PRACH procedure.
Referring to FIG. 2, because more than two UEs 201 have transmitted the same preamble in step 211, both receive an ACK for the transmitted preamble in step 212, and a collision may occur even in an RRC CONNECTION REQUEST message transmission process of step 221. If the RRC CONNECTION REQUEST message transmission fails due to the collision between the messaging of the UEs 201, an NB 202 does not transmit the RRC CONNECTION SETUP message corresponding to step 122 of FIG. 1. The UEs 201 activate a timer 222 from the time they transmit the RRC CONNECTION REQUEST message of step 221, and perform a new RACH procedure of step 231 upon a failure to receive the RRC CONNECTION SETUP message until a predetermined time t0 has elapsed. In this case, however, the UEs 201 may enter a new RACH procedure after waiting the predetermined time t0, causing a considerable delay in initial access.